Lithographic printing is based on the immiscibility of oil and water, wherein the oily material or the printing ink is preferably accepted by the image area, and the water or fountain solution is preferably accepted by the non-image area. When an appropriately produced surface is moistened with water and a printing ink is applied, the background or non-image area accepts the water and repels the printing ink, while the image area accepts the printing ink and repels the water. The printing ink in the image area is then transferred to the surface of a material such as paper, fabric and the like, on which the image is to be formed. Generally, however, the printing ink is first transferred to an intermediate material, referred to as blanket, which then in turn transfers the printing ink onto the surface of the material on which the image is to be formed; this technique is referred to as offset lithography.
A frequently used type of lithographic printing plate precursor comprises a photosensitive coating applied onto a substrate on aluminum basis. The coating can react to radiation such that the exposed portion becomes so soluble that it is removed during the developing process. Such a plate is referred to as positive working. On the other hand, a plate is referred to as negative working if the exposed portion of the coating is hardened by the radiation. In both cases, the remaining image area accepts printing ink, i.e. is oleophilic, and the non-image area (background) accepts water, i.e. is hydrophilic. The differentiation between image and non-image areas takes place during exposure, for which a film is attached to the printing plate precursor under vacuum in order to guarantee good contact. The plate is then exposed by means of a radiation source part of which is comprised of UV radiation. When a positive plate is used, the area on the film corresponding to the image on the plate is so opaque that the light does not reach the plate, while the area on the film corresponding to the non-image area is clear and allows light to permeate the coating, whose solubility increases. In the case of a negative plate, the opposite takes place: The area on the film corresponding to the image on the plate is clear, while the non-image area is opaque. The coating beneath the clear film area is hardened (for instance by photopolymerization) due to the incident light, while the area not affected by the light is removed during developing. The light-hardened surface of a negative working plate is therefore oleophilic and accepts printing ink, while the non-image area that used to be coated with the coating removed by the developer is desensitized and therefore hydrophilic.
Alternatively, the plate can also be imagewise exposed digitally without a film e.g. by lasers. According to recent developments, plate precursors with heat-sensitive layers are used wherein by imagewise direct heating or irradiation with IR radiation that is converted into heat, differences in the developer solubility of the heated and unheated areas of the coating are generated.
Sometimes water-soluble polymers are used as temporary coatings (sometimes called “overcoat”) on lithographic printing plate precursors having an oxygen-sensitive coating like a photopolymerizable coating. The water-soluble polymers have the function of protecting the coating from atmospheric oxygen during storage, exposure and in particular during the time between exposure and further processing (development and the like). During that time period the temporary coating has to exhibit a sufficient adhesion to the light-sensitive substrate so that safe handling (manufacture, packing, transport, exposure etc.) is guaranteed without tearing of the layers. Prior to the development the overcoat can is removed, preferably by washing with water or the overcoat is dissolved within the developer together with the non-image parts.
Usually, a substrate, in particular an aluminum substrate with aluminum oxide layer, is provided with a hydrophilic protective layer (also referred to as “interlayer”) before the radiation-sensitive coating is applied. This hydrophilic layer improves the water acceptance of the (non-printing) background areas of a lithographic printing plate and improves the repulsion of the printing ink in these areas. A suitable hydrophilic protective layer also ensures that during developing the soluble portions of the radiation-sensitive layer are removed easily and residue-free from the substrate so that clean background areas are obtained during printing. Without such a residue-free removal, what is referred to as toning would occur during printing, i.e. the background areas would accept printing ink.
For a clean printed image it is necessary that the image areas (i.e. the imagewise remaining coating) accept the printing ink well, while the non-image areas (i.e. the imagewise revealed substrate, such as e.g. an aluminum substrate) should not accept the printing ink. In order to protect the imagewise revealed substrate, such as e.g. an aluminum substrate, against fingerprints, the formation of aluminum oxide, corrosion and mechanical attacks, such as scratches, when the printing plate is mounted in the printing machine, i.e. to maintain and possibly improve the hydrophilic nature of the non-image areas, the developed printing plate is usually subjected to a “gumming” treatment (also referred to as “finishing”). Gumming the plate prior to storing it or before long periods of standstill of the printing machine ensures that the non-image areas remain hydrophilic. When printing is started, the gumming solution has to be able to be washed off the plate quickly with the fountain solution so that the image areas are capable of accepting ink immediately. Gumming solutions have been known for a long time and are for instance disclosed in DE 29 26 645, DE 20 42 217 A1, U.S. Pat. No. 4,880,555 A1, U.S. Pat. No. 4,033,919 A1, and U.S. Pat. No. 4,162,920 A1.
DE 25 04 594 A1 discloses a gumming solution comprising phosphoric acid and a copolymer containing structural units derived from acryl amide and structural units having carboxylic groups. This gumming solution is not suitable for lithographic substrates having high adhesion to hydrophobic coatings (e.g. substrates without interlayer) because toning on press is observed for such plates.
DE 29 25 363 A1 describes a gumming composition comprising an aqueous phase and an oily phase, wherein the aqueous phase contains a hydrophilic polymer and the oily phase contains an organic solvent, a nonionic alkyl phenyl surfactant with HLB<14, an aliphatic acid ester surfactant and an anionic surfactant. While this composition provides good desensibilisation to plates with interlayer, it is not suitable for lithographic substrates having high adhesion to hydrophobic coatings (e.g. substrates without interlayer) because for such plates toning is observed. Another emulsion type surface protective agent for litho plates is disclosed in EP 943 967 A2; it is also not suitable for plates without interlayer due to toning problems.
Also the gumming compositions disclosed in U.S. Pat. Nos. 4,143,021; 414,531; 4,266,481; and U.S. Pat. No. 5,736,256 are not suitable for lithographic substrates having high adhesion to hydrophobic coatings (e.g. substrates without interlayer) because toning on press is observed.
In EP 985 546 A1a surface protecting agent for litho plates is disclosed which is a specific copolymer of ethylene oxide and propylene oxide. The use of such an agent in a gumming solution cannot prevent toning during printing with lithographic substrates having high adhesion to hydrophobic coatings (e.g. substrates without interlayer).
EP 1 199 606 A1 discloses a process for making a negative working lithographic printing plate wherein the developing solution contains an inorganic alkali agent and a nonionic surface active agent having a polyoxyalkylene ether group. It was found that the clean up behavior of a lithographic substrate having high adhesion to hydrophobic coating (e.g. substrate without interlayer) is insufficient and strong toning was observed on press if such developing solution is used.
EP 1 600 820 A1 describes a method for processing planographic printing plate comprising (a) developing the imagewise exposed plate with a developer containing no or only up to 0.1 wt. % silicate, and (b) treating the developed plate with a protection solution containing a phosphonic acid compound at a temperature from 40 to 90° C.
EP 1 260 866 A2 discloses a method for processing an imagewise exposed lithographic printing plate comprising (a) applying a developer, and (b) rinsing the developed plate with rinse water comprising a water-soluble film-forming polymer and a phosphoric acid derivative. This method avoids an additional gumming step, however, it is not suitable for plates without hydrophilic interlayer since toning on press is observed.
DE 25 30 502 A1 describes a method for simultaneously developing and gumming printing plates, wherein the liquid used comprises besides water, a small amount of organic solvent, a water-soluble colloid like dextrin and a salt. This method, however, only works for plates having a hydrophilic interlayer.
In WO 2006/056439 A1 polymers comprising amino groups modified by acid groups and their use in fountain solutions used in the lithographic printing process are described.
For excellent performance of lithographic printing plates it is necessary to have both good hydrophilic properties of the substrate and good adhesion of the image areas (of the coating) to the substrate. Good hydrophilic properties of the substrate are necessary in order to avoid toning during printing. Therefore, very often an interlayer is applied to the substrate for improving hydrophilicity of the substrate. Unfortunately, interlayers usually deteriorate the adhesion between the printing image areas and the substrate which in turn decreases the print run length.
For improving the quality of the printed image (e.g. the image resolution) FM screens (FM=frequency-modulated) with 30 μm dot size or less have been developed where the dots become very small in size which then results in worse adhesion. For plate precursors to be imaged with FM screens, it is therefore important that there is excellent adhesion; it is, however, desired that nevertheless toning does not become an issue (i.e. the hydrophilicity of the substrate is excellent).